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Full text of "The Bakerian lecture [electronic resource]: on the theory of light and colours"

YALE 
MEDICAL LIBRARY 




HISTORICAL 
LIBRARY 



EX LIBRIS 
JOHN FARQUHAR FULTON 



V* 




THE 
ABNER WELLBORN 
MEDICAL LIBI 
1923 




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m 



C iO 



II. The Bakerian Lecture. On the Theory of Light and Colours. 
By Thomas Young, M. D. F. R. S. Professor of Natural Phi- 
losophy in the Royal Institution. 



Read November 12, 1801. 

Although the invention of plausible hypotheses, independent 
of any connection with experimental observations, can be of 
very little use in the promotion of natural knowledge ; yet the 
discovery of simple and uniform principles, by which a great 
number of apparently heterogeneous phenomena are reduced 
to coherent and universal laws, must ever be allowed to be ef 
considerable importance towards the improvement of the human 
intellect. 

The object of the present dissertation is not so much to pro- 
pose any opinions which are absolutely new, as to refer some 
theories, which have been already advanced, to their original 
inventors, to support them by additional evidence, and to apply 
them to a great number of diversified facts, which have hitherto 
been buried in obscurity. Nor is it absolutely necessary in this 
instance to produce a single new experiment; for of experi- 
ments there is already an ample store, which are so much the 
more unexceptionable, as they must have been conducted with- 
out the least partiality for the system by which they will be 
explained; yet some facts, hitherto unobserved, will be brought 
forwards, in order to show the perfect agreement of that system 
with the multifarious phenomena of nature. 






Dr. Young's Lecture, &c. 13 

The optical observations of Newton are yet unrivalled ; and, 
excepting some casual inaccuracies, they only rise in our esti- 
mation, as we compare them with later attempts to improve 
on them. A further consideration of the colours of thin plates, 
as they are described in the second book of Newton's optics, 
has converted that prepossession which I before entertained for 
the undulatory system of light, into a very strong conviction of 
its truth and sufficiency; a conviction which has been since most 
strikingly confirmed, by an analysis of the colours of striated 
substances. The phenomena of thin plates are indeed so sin- 
gular, that their general complexion is not without great diffi- 
culty reconcileable to any theory, however complicated, that 
has hitherto been applied to them ; and some of the principal 
circumstances have never been explained by the most gratuitous 
assumptions ; but it will appear, that the minutest particulars of 
these phenomena, are not only perfectly consistent with the 
theory which will now be detailed, but that they are all the 
necessary consequences of that theory, without any auxiliary 
suppositions ; and this by inferences so simple, that they be- 
come particular corollaries, which scarcely require a distinct 
enumeration. 

A more extensive examination of Newton's various writings 
has shown me, that he was in reality the first that suggested 
such a theory as I shall endeavour to maintain ; that his own 
opinions varied less from this theory than is now almost uni- 
versally supposed ; and that a variety of arguments have been 
advanced, as if to confute him, which may be found nearly in 
a similar form in his own works ; and this by no less a mathe- 
matician than Leonard Euler, whose system of light, as far 
as it is worthy of notice, either was, or might have been, 



14 Dr. Young's Lecture on 

wholly borrowed from Newton, Hooke, Huygens, and Male- 

BRANCHE. 

Those who are attached, as they may be with the greatest 
justice, to every doctrine which is stamped with the Newtonian 
approbation, will probably be disposed to bestow on these con- 
siderations so much the more of their attention, as they appear 
to coincide more nearly with Newton's own opinions. For 
this reason, after having briefly stated each particular position 
of my theory, I shall collect, from Newton's various writings, 
such passages as seem to be the most favourable to its admis- 
sion ; and, although I shall quote some papers which may be 
thought to have been partly retracted at the publication of the 
optics, yet I shall borrow nothing from them that can be sup- 
posed to militate against his maturer judgment. 

HYPOTHESIS I. 

A luminijerous Ether pervades the Universe, rare and elastic in a 
high degree. 

Passages from Newton. 

" The hypothesis certainly has a much greater affinity with 
" his own," that is, Dr. Hooke's, " hypothesis, than he seems 
" to be aware of; the vibrations of the ether being as useful and 
" necessary in this, as in his." (Phil. Trans. Vol. VII. p. 5087. 
Abr. Vol. I. p. 145. Nov. 1672.) 

" To proceed to the hypothesis : first, it is to be supposed 
" therein, that there is an ethereal medium, much of the same 
" constitution with air, but far rarer, subtler, and more strongly 
" elastic. — It is not to be supposed, that this medium is one 
" uniform matter, but compounded, partly of the main phleg- 
" matic body of ether, partly of other various ethereal spirits, 



the Theory of Light and Colours. 15 

" much after the manner that air is compounded of the phleg- 
" matic body of air, intermixed with various vapours and 
" exhalations : for the electric and magnetic effluvia, and gravi- 
" tating principle, seem to argue such variety." (Birch: Hist, of 
R. S. Vol. III. p. 249. Dec. 1675.) 

" Is not the heat (of the warm room) conveyed through the 
" vacuum by the vibrations of a much subtiler medium than air? 
" — And is not this medium the same with that medium by which 
" light is refracted and reflected, and by whose vibrations light 
" communicates heat to bodies, and is put into fits of easy re- 
" flection, and easy transmission ? And do not the vibrations of 
" this medium in hot bodies, contribute to the intenseness and 
" duration of their heat ? And do not hot bodies communicate 
" their heat to contiguous cold ones, by the vibrations of this me- 
" dium propagated from them into the cold ones ? And is not this 
" medium exceedingly more rare and subtile than the air, and 
" exceedingly more elastic and active ? And doth it not readily 
" pervade all bodies ? And is it not, by its elastic force, expanded 
" through all the heavens ? — May not planets and comets, and 
" all gross bodies, perform their motions in this ethereal me- 
" dium ? — And may not its resistance be so small, as to be 
" inconsiderable? For instance, if this ether (for so I will call 
" it) should be supposed 700,000 times more elastic than our 
" air, and above 700,000 times more rare, its resistance would 
" be about 600,000000 times less than that of water. And 
" so small a resistance would scarce make any sensible altera- 
" tion in the motions of the planets, in ten thousand years. 
" If any one would ask how a medium can be so rare, let him 
" tell me — how an electric body can by friction emit an exha- 
" lation so rare and subtile, and yet so potent ? — And how the 



10> Dr. Young's Lecture on 

" effluvia of a magnet can pass through a plate of glass, with- 
" out resistance, and yet turn a magnetic needle beyond the 
"glass?" (Optics, Qu. 18, 22.) 

HYPOTHESIS II. 

Undulations are excited in this Ether whenever a Body becomes 
luminous. 
Scholium. I use the word undulation, in preference to vibra- 
tion, because vibration is generally understood as implying a 
motion which is continued alternately backwards and forwards, 
by a combination of the momentum of the body with an acce- 
lerating force, and which is naturally more or less permanent ; 
but an undulation is supposed to consist in a vibratory motion, 
transmitted successively through different parts of a medium, 
without any tendency in each particle to continue its motion, 
except in consequence of the transmission of succeeding undu- 
lations, from a distinct vibrating body ; as, in the air, the vibra- 
tions of a chord produce the undulations constituting sound.. 

Passages from Newton. 

" Were I to assume an hypothesis, it should be this, if pro- 
" pounded more generally, so as not to determine what light is, 
" further than that it is something or other capable of exciting 
" vibrations in the ether ; for thus it will become so general and 
" comprehensive of other hypotheses, as to leave little room for 
" new ones to be invented." (Birch. Vol. III. p. 249. Dec. 1675.) 

" In the second place, it is to be supposed, that the ether is a 
" vibrating medium like air, only the vibrations far more swift 
" and minute ; those of air, made by a man's ordinary voice, 
» succeeding one another at more than half a foot, or a foot 



the Theory of Light and Colours. 1 7 

" distance ; but those of ether at a less distance than the hun- 
" dred thousandth part of an inch. And, as in air the vibra- 
" tions are some larger than others, but yet all equally swift, 
" (for in a ring of bells the sound of every tone is heard at two 
" or three miles distance, in the same order that the bells are 
" struck,) so, I suppose, the ethereal vibrations differ in big- 
" ness, but not in swiftness. Now, these vibrations, beside their 
" use in reflection and refraction, may be supposed the chief 
" means by which the parts of fermenting or putrifying sub- 
" stances, fluid liquors, or melted, burning, or other hot bodies, 
" continue in motion." (Birch Vol. III. p. 251. Dec. 1675.) 

" When a ray of light falls upon the surface of any pellucid 
" body, and is there refracted or reflected, may not waves of 
" vibrations, or tremors, be thereby excited in the refracting or 
" reflecting medium ? — And are not these vibrations propagated 
" from the point of incidence to great distances ? And do they 
" not overtake the rays of light, and by overtaking them'suc- 
" cessively, do not they put them into the fits of easy reflection 
" and easy transmission described above ?" (Optics. Ou. 17.) 

" Light is in fits of easy reflection and easy transmission, 
" before its incidence on transparent bodies. And probably it is 
" put into such fits at its first emission from luminous bodies, 
" and continues in them during all its progress." (Optics. 
Second Book. Part III. Prop. 13.) 



l8 Dr. Young's Lecture on 

HYPOTHESIS III. 

The Sensation of different Colours depends on the different fre- 
quency of Vibrations, excited by Light in the Retina. 

Passages from Newton. 
" The objector's hypothesis, as to the fundamental part of it, 
" is not against me. That fundamental supposition is, that the 
" parts of bodies, when briskly agitated, do excite vibrations in 
" the ether, which are propagated every way from those bodies 
" in straight lines, and cause a sensation of light by beating 
" and dashing against the bottom of the eye, something after 
" the manner that vibrations in the air cause a sensation of 
" sound by beating against the organs of hearing. Now, the 
" most free and natural application of this hypothesis to the 
" solution of phenomena, I take to be this : that the agitated 
" parts of bodies, according to their several sizes, figures, and 
" motions, do excite vibrations in the ether of various depths 
" or bignesses, which, being promiscuously propagated through 
'* that medium to our eyes, effect in us a sensation of light of a 
" white colour ; but if by any means those of unequal bignesses 
" be separated from one another, the largest beget a sensation 
" of a red colour, the least or shortest of a deep violet, and 
" the intermediate ones of intermediate colours ; much after 
" the manner that bodies, according to their several sizes, 
" shapes, and motions, excite vibrations in the air of various 
" bignesses, which, according to those bignesses, make several 
" tones in sound : that the largest vibrations are best able to 
" overcome the resistance of a refracting superficies, and so 
" break through it with least refraction; whence the vibrations 



the Theory of Light and Colours. ig 

" of several bignesses, that is, the rays of several colours, which 
" are blended together in light, must be parted from one an- 
" other by refraction, and so cause the phenomena of prisms, 
" and other refracting substances ; and that it depends on the 
" thickness of a thin transparent plate or bubble, whether a 
" vibration shall be reflected at its further superficies, or trans- 
" mitted; so that, according to the number of vibrations, inter- 
" ceding the two superficies, they may be reflected or transmitted 
" for many successive thicknesses. And, since the vibrations 
" which make blue and violet, are supposed shorter than those 
" which make red and yellow, they must be reflected at a less 
" thickness of the plate : which is sufficient to explicate all the 
" ordinary phenomena of those plates or bubbles, and also of 
" all natural bodies, whose parts are like so many fragments of 
" such plates. These seem to be the most plain, genuine, and 
" necessary conditions of this hypothesis. And they agree so 
"justly with my theory, that if the animadversor think fit to 
" apply them, he need not, on that account, apprehend a divorce 
" from it. But yet, how he will defend it from other difficulties, 
" I know not." (Phil. Trans. Vol. VII. p. 5088. Abr. Vol. I. 
p. 145. Nov. 1672.) 

" To explain colours, I suppose, that as bodies of various 
" sizes, densities, or sensations, do by percussion or other 
" action excite sounds of various tones, and consequently vi- 
" brations in the air of different bigness ; so the rays of light, 
" by impinging on the stiff refracting superficies, excite vibra- 
" tions in the ether, — of various bigness ; the biggest, strongest, 
" or most potent rays, the largest vibrations ; and others shorter, 
" according to their bigness, strength, or power: and therefore 
" the ends of the capillamenta of the optic nerve, which pave 
Da 



20 Or- Young's Lecture on 

" or face the retina, being such refracting superficies, when the 
" rays impinge upon them, they must there excite these vibra- 
" tions, which vibrations (like those of sound in a trunk or 
" trumpet) will run along the aqueous pores or crystalline pith 
" of the capillamenta, through the optic nerves, into the senso- 
" rium ; — and there, I suppose, affect the sense with various 
" colours, according to their bigness and mixture ; the biggest 
" with the strongest colours, reds and yellows ; the least with 
" the weakest, blues and violets ; the middle with green ; and a 
" confusion of all with white, much after the manner that, in 
" the sense of hearing, nature makes use of aerial vibrations of 
" several bignesses, to generate sounds of divers tones ; for the 
" analogy of nature is to be observed." (Birch Vol, III. p. 262. 
Dec. 1675.) 

" Considering the lastingness of the motions excited in the 
" bottom of the eye by light, are they not of a vibrating nature ? 
" — Do not the most refrangible rays excite the shortest vibra- 
" tions, — the least refrangible the largest ? May not the harmony 
" and discord of colours arise from the proportions of the vibra- 
" tions propagated through the fibres of the optic nerve into 
" the brain, as the harmony and discord of sounds arise from 
*' the proportions of the vibrations of the air ?" (Optics, Ou. 
16, 13, 14.) 

Scholium. Since, for the reason here assigned by Newton, 
it is probable that the motion of the retina is rather of a vibra- 
tory than of an undulatory nature, the frequency of the vibrations 
must be dependent on the constitution of this substance. Now, 
as it is almost impossible to conceive each sensitive point of the 
retina to contain an infinite number of particles, each capable 
of vibrating in perfect unison with every possible undulation, it 



■*- 



the Theory of Light and Colours. 21 

becomes necessary to suppose the number limited, for instance, 
to the three principal colours, red, yellow, and blue, of which 
the undulations are related in magnitude nearly as the numbers 
8, 7, and 6 ; and that each of the particles is capable of being 
put in motion less or more forcibly, by undulations differing 
less or more from a perfect unison ; for instance, the undula- 
tions of green light being nearly in the ratio of 6\, will affect 
equally the particles in unison with yellow and blue, and pro- 
duce the same effect as a light composed of those two species : 
and each sensitive filament of the nerve may consist of three 
portions, one for each principal colour. Allowing this statement, 
it appears that any attempt to produce a musical effect from 
colours, must be unsuccessful, or at least that nothing more 
than a very simple melody could be imitated by them ; for the 
period, which in fact constitutes the harmony of any concord, 
being a multiple of the periods of the single undulations, would 
in this case be wholly without the limits of sympathy of the 
retina, and would lose its effect; in the same manner as the 
harmony of a third or a fourth is destroyed, by depressing it to 
the lowest notes of the audible scale. In hearing, there seems 
to be no permanent vibration of any part of the organ. 

HYPOTHESIS IV. 

All material Bodies have an Attraction for the ethereal Medium, 
by means of which it is accumulated within their Substance, and 
for a small Distance around them, in a State of greater Density, 
but not of greater Elasticity. 

It has been shewn, that the three former hypotheses, which 
may be called essential, are literally parts of the .more compli- 
cated Newtonian system. This fourth hypothesis differs perhaps 






gs Dr. Young's Lecture on 

in some degree from any that have been proposed by former 
authors, and is diametrically opposite to that of Newton ; but, 
both being in themselves equally probable, the opposition is 
merely accidental ; and it is only to be inquired which is the 
best capable of explaining the phenomena. Other suppositions 
might perhaps be substituted for this, and therefore I do not 
consider it as fundamental, yet it appears to be the simplest and 
best of any that have occurred to me. 

PROPOSITION I. 

All Impulses are propagated in a homogeneous elastic Medium 
with an equable Velocity. 
Every experiment relative to sound coincides with the obser- 
vation already quoted from Newton, that all undulations are 
propagated through the air with equal velocity; and this is 
further confirmed by calculations. (Lagrange. Misc. Taur. 
Vol. I. p. 91. Also, much more concisely, in my Syllabus of a 
course of Lectures on Natural and Experimental Philosophy, 
about to be published. Article 289. ) If the impulse be so great 
as materially to disturb the density of the medium, it will be no 
longer homogeneous ; but, as far as concerns our senses, the 
quantity of motion may be considered as infinitely small. It is 
surprising that Euler, although aware of the matter of fact, 
should still have maintained, that the more frequent undulations 
are more rapidly propagated. (Theor. mus. and Conject. phys.) 
It is possible, that the actual velocity of the particles of the 
luminiferous ether may bear a much less proportion to the velo- 
city of the undulations than in sound ; for light may be excited 
' by the motion of a body moving at the rate of only one mile 
in the time that light moves a hundred millions. 



the Theory of Light and Colours. 23 

Scholium 1. It has been demonstrated, that in different 
mediums the velocity varies in the subduplicate ratio of the 
force directly, and of the density inversely. ( Misc. Taur. Vol. I. 
p. 91. Young's Syllabus. Art. 294.) 

Scholium 2. It is obvious, from the phenomena of elastic 
bodies and of sounds, that the undulations may cross each other 
without interruption. But there is no necessity that the various 
colours of white light should intermix their undulations ; for, 
supposing the vibrations of the retina to continue but a five hun^- 
dredth of a second after their excitement, a million undulations 
of each of a million colours may arrive in distinct succession 
within this interval of time, and produce the same sensible 
effect, as if all the colours arrived precisely at the same instant. 

PROPOSITION II. 

An Undulation conceived to originate from the Vibration of a 
single Particle, must expand through a homogeneous Medium 
in a spherical Form, but with different quantities of Motion in. 
different Parts. 

For, since every impulse, considered as positive or negative, 
is propagated with a constant velocity, each part of the undu- 
lation must in equal times have past through equal distances 
from the vibrating point. And, supposing the vibrating particle, 
in the course of its motion, to proceed forwards to a small dis- 
tance in a given direction, the principal strength of the undula- 
tion will naturally be straight before it ; behind it, the motion 
will be equal, in a contrary direction ; and, at right angles to 
the line of vibration, the undulation will be evanescent. 

Now, in order that such an undulation may continue its pro- 
gress to any considerable distance, there must be in each part 
of it, a tendency to, preserve its own motion in a right line from 



2 a Dr. Young's Lecture on 

the centre ; for, if the excess of force at any part were commu- 
nicated to the neighbouring particles, there can be no reason 
why it should not very soon be equalised throughout, or, in 
other words, become wholly extinct, since the motions in con- 
trary directions would naturally destroy each other. The 
origin of sound from the vibration of a chord is evidently of 
this nature ; on the contrary, in a circular wave of water, every 
part is at the same instant either elevated or depressed. It may 
be difficult to show mathematically, the mode in which this 
inequality of force is preserved ; but the inference from the 
matter of fact, appears to be unavoidable ; and, while the science 
of hydrodynamics is so imperfect that we cannot even solve the 
simple problem of the time required to empty a vessel by a 
given aperture, it cannot be expected that we should be able to 
account perfectly for so complicated a series of phenomena, as 
those of elastic fluids. The theory of Huygens indeed explains 
the circumstance in a manner tolerably satisfactory : he sup- 
poses every particle of the medium to propagate a distinct un- 
dulation in all directions ; and that the general effect is only 
perceptible where a portion of each undulation conspires in 
direction at the same instant ; and it is easy to show that such a 
general undulation would in all cases proceed rectilinearly, with 
proportionate force ; but, upon this supposition, it seems to 
follow, that a greater quantity of force must be lost by the 
divergence of the partial undulations, than appears to be con- 
sistent with the propagation of the effect to any considerable 
distance. Yet it is obvious, that some such limitation of the 
motion must naturally be expected to take place ; for, if the 
intensity of the motion of any particular part, instead of conti- 
nuing to be propagated straight forwards, were supposed to 
affect the intensity of a neighbouring part of the undulation, an 



the Theory of Light and Colours. 25 

impulse must then have travelled from an internal to an exter- 
nal circle in an oblique direction, in the same time as in the 
direction of the radius, and consequently with a greater velo- 
city; against the first proposition. In the case of water, the 
velocity is by no means so rigidly limited as in that of an 
elastic medium. Yet it is not necessary to suppose, nor is it 
indeed probable, that there is absolutely not the least lateral 
communication of the force of the undulation, but that, in highly 
elastic mediums, this communication is almost insensible. In 
the air, if a chord be perfectly insulated, so as to propagate 
exactly such vibrations as have been described, they will in 
fact be much less forcible than if the chord be placed in 
the neighbourhood of a sounding board, and probably in some 
measure because of this lateral communication of motions of an 
opposite tendency. And the different intensity of different parts 
of the same circular undulation may be observed, by holding a 
common tuning fork at arm's length, while sounding, and 
turning it, from a plane directed to the ear, into a position per- 
pendicular to that plane. 

proposition ni. 
A Portion of a spherical Undulation, admitted through an Aper- 
ture into a quiescent Medium, will proceed to be further propa- 
gated rectilinearly in concentric Superficies, terminated laterally 
by weak and irregular Portions of newly diverging Undula- 
tions. 

At the instant of admission, the circumference of each of the 

undulations may be supposed to generate a partial undulation, 

filling up the nascent angle between the radii and the surface 

terminating the medium ; but no sensible addition will be made 

mdcccii. E 



26 Dr. Young's Lecture on 

to its strength by a divergence of motion from any other parts 
of the undulation, for want of a coincidence in time, as has 
already been explained with respect to the various force of a 
spherical undulation. If indeed the aperture bear but a. small 
proportion to the breadth of an undulation, the newly generated 
undulation may nearly absorb the whole force of the portion 
admitted ; and this is the case considered by Newton in the 
Principia. But no experiment can be made under these circum- 
stances with light, on account of the minuteness of its undula- 
tions, and the interference of inflection ; and yet some faint 
radiations do actually diverge beyond any probable limits of 
inflection, rendering the margin of the aperture distinctly visible 
in all directions ; these are attributed by Newton to some un- 
known cause, distinct from inflection; (Optics, Third Book, 
Obs. 5. ) and they fully answer the description of this propo- 
sition. 

Let the concentric lines in Fig. 1 . (Plate I.) represent the con- 
temporaneous situation of similar parts of a number of suc- 
cessive undulations diverging from the point A ; they will also 
represent the successive situations of each individual undulation: 
let the force of each undulation be represented by the breadth of 
the line, and let the cone of light ABC be admitted through 
the aperture BC ; then the principal undulations will proceed 
in a rectilinear direction towards GH, and the faint radiations 
on each side will diverge from B and C as centres, without 
receiving any additional force from any intermediate point D 
of the undulation, on account of the inequality of the lines DE 
and DF. But, if we allow some little lateral divergence from 
the extremities of the undulations, it must diminish their force, 
without adding materially to that of the dissipated light; and their 



the Theory of Light and Colours. 27 

termination, instead of the right line BG, will assume the form 
CH; since the loss of force must be more considerable near to C 
than at greater distances. This line corresponds with the boun-r 
dary of the shadow in Newton's first observation, Fig. 1; and 
it is much more probable that such a dissipation of light was 
the cause of the increase of the shadow in that observation, 
than that it was owing to the action of the inflecting atmo- 
sphere, which must have extended a thirtieth of an inch each 
way in order to produce it ; especially when it is considered 
that the shadow was not diminished by surrounding the hair 
with a denser medium than air, which must in all probability 
have weakened and contracted its inflecting atmosphere. In 
other circumstances, the lateral divergence might appear to in- 
crease, instead of diminishing, the breadth of the beam. 

As the subject of this proposition has always been esteemed 
the most difficult part of the undulatory system, it will be 
proper' to examine here the objections which Newton has 
grounded upon it. 

'* To me, the fundamental supposition itself seems impossible ; 
" namely, that the waves or vibrations of any fluid can, like the 
" rays of light, be propagated in straight lines, without a con- 
" tinual and very extravagant spreading and bending every 
" way into the quiescent medium, where they are terminated 
'* by it. I mistake, if there be not both experiment and demon- 
" stration to the contrary." (Phil. Trans. VII. 5089, Abr. I. 
146. Nov. 1672.) 

" Motus omnis per fluidum propagatus divergit a recto tra- 
" mite in spatia immota." 

" Quoniam medium ibi," in the middle of an undulation 
E 2 



2 g Dr. Young's Lecture on 

admitted, " densius est, quam in spatiis hinc inde, dilatabit sese 
" tarn versus spatia utrinque sita, quam versus pulsuum rariora 
« intervalla; eoque pacto— pulsus eadem fere celeritate sese in 
"medii partes quiescentes hinc inde relaxare debent ;— ideoque 
« spatium totum occupabunt— Hoc experimur in sonis." (Prin- 
cip. Lib. II. Prop. 43. 

" Are not all hypotheses erroneous, in which light is supposed 
" to consist in pression or motion, propagated through a fluid 
" medium ?— If it consisted in pression or motion, propagated 
" either in an instant, or in time, it would bend into the shadow. 
" For pression or motion cannot be propagated in a fluid in 
« right lines beyond an obstacle which stops part of the motion, 
" but will bend and spread every way into the quiescent medium 
« which lies beyond the obstacle.— The waves on the surface of 
" stagnating water, passing by the sides of a broad obstacle 
" which stops part of them, bend afterwards, and dilate them- 
" selves gradually into the quiet water behind the obstacle. 
" The waves, pulses, or vibrations of the air, wherein sounds 
" consist, bend manifestly, though not so much as the waves 
" of water. For a bell or a cannon may be heard beyond a 
"hill, which intercepts the sight of the sounding body; and 
" sounds are propagated as readily through crooked pipes as 
" straight ones. But light is never known to follow crooked 
" passages, nor to bend into the shadow. For the fixed stars, 
« by the interposition of any of the planets, cease to be seen. 
" And so do the parts of the sun, by the interposition of the 
" moon, Mercury, or Venus. The rays which pass very near 
" to the edges of any body, are bent a little by the action of the 
" body ; — but this bending is not towards but from the shadow, 



the Theory of Light and Colours. 29 

<• and is performed only in the passage of the ray by the body, 
« and at a very small distance from it. So soon as the ray is 
« past the body, it goes right on." (Optics, Qu. 28.) 

Now the proposition quoted from the Principia does not di- 
rectly contradict this proposition; for it does not assert that 
such a motion must diverge equally in all directions; neither 
can it with truth be maintained, that the parts of an elastic me- 
dium communicating any motion, must propagate that motion 
equally in all directions. (Phil. Trans, for 1800. p. 109— 112.) 
All that can be inferred by reasoning is, that the marginal 
parts of the undulation must be somewhat weakened, and that 
there must be a faint divergence in every direction; but whe- 
ther either of these effects might be of sufficient magnitude to 
be sensible, could not have been inferred from argument, if the 
affirmative had not been rendered probable by experiment. 

As to the analogy with other fluids, the most natural inference 
from it is this : " The waves of the air, wherein sounds consist, 
« bend manifestly, though not so much as the waves of water ;" 
water being an inelastic, and air a moderately elastic medium ; 
but ether being most highly elastic, its waves bend very far less 
than those of the air, and therefore almost imperceptibly. 
Sounds are propagated through crooked passages, because their 
sides are capable of reflecting sound, just as light would be pro- 
pagated through a bent tube, if perfectly polished within. 

The light of a star is by far too weak to produce, by its faint 
divergence, any visible illumination of the margin of a planet 
eclipsing it ; and the interception of the sun's light by the moon, 
is as foreign to the question, as the statement of inflectwn is 
inaccurate. 

To the argument adduced by Huygens, in favour of the 



o Dr. Young's Lecture on 

rectilinear propagation of undulations, Newton has made no 
reply ; perhaps because of his own misconception of the nature of 
the motions of elastic mediums, as dependent on a peculiar law 
of vibration, which has been corrected by later mathematicians. 
(Phil. Trans, for 1800, p. 116.) On the whole, it is presumed, 
that this proposition may be safely admitted, as perfectly con- 
sistent with analogy and with experiment. 

PROPOSITION IV. 

When an Undulation arrives at a Surface which is the Limit of 
Mediums of different Densities, a partial Reflection takes place, 
proportionate in Force to the Difference of the Densities. 
This may be illustrated, if not demonstrated, by the analogy 
of elastic bodies of different sizes. " If a smaller elastic body 
" strikes against a larger one, it is well known that the smaller 
" is reflected more or less powerfully, according to the diffe- 
" rence of their magnitudes : thus, there is always a reflection 
" when the rays of light pass from a rarer to a denser stratum 
" of ether ; and frequently an echo when a sound strikes 
*' against a cloud. A greater body striking a smaller one, pro- 
" pels it, without losing all its motion : thus, the particles of a 
" denser stratum of ether, do not impart the whole of their 
" motion to a rarer, but, in their effort to proceed, they are 
" recalled by the attraction of the refracting substance with 
" equal force ; and thus a reflection is always secondarily pro- 
" duced, when the rays of light pass from a denser to a rarer 
" stratum." (Phil. Trans, for 1800. p. 127 .) But it is not ab- 
solutely necessary to suppose an attraction in the latter case, 
since the effort to proceed would be propagated backwards 
without it, and the undulation would be reversed, a rarefaction 



the Theory of Light and Colours. 31 

returning in place of a condensation ; and this will perhaps be 
found most consistent with the phenomena. 

proposition v. 

When an Undulation is transmitted through a Surface terminating 
different Mediums, it proceeds in such a Direction, that the Sines 
of the Angles of Incidence and Refraction are in the constant 
Ratio of the Velocity of Propagation in the two Mediums. 

- (Barrow, Lect. Opt. II. p. 4. Huygens, de la Lum. cap. 3. 

Euler, Conj. Phys. Phil. Trans, for 1800, p. 128. Young's 

Syllabus. Art. 382.) 

Corollary 1. The same demonstrations prove the equality of 

the angles of reflection and incidence. 

Corollary 2. It appears from experiments on the refraction of 

condensed air, that the ratio of the difference of the sines varies 

simply as the density. Hence it follows, by Schol. I. Prop. I. 

that the excess of the density of the ethereal medium is in the 

duplicate ratio of the density of the air ; each particle cooperating 

with its neighbours in attracting a greater portion of it. 

PROPOSITION VI. 

When an Undulation falls' on the Surface of a rarer Medium, so 
obliquely that it cannot be regularly refracted, it is totally re- 
flected, at an Angle equal to that of its Incidence. 
(Phil. Trans, for 1800, p. 128.) 

Corollary. This phenomenon tends to prove the gradual in- 
crease and diminution of density at the surface terminating two 
mediums, as supposed in hypothesis iv ; although Huygens 
has attempted to explain it somewhat differently. 



o 2 Dr. Young's Lecture on 

PROPOSITION VII. 

If equidistant Undulations be supposed to pass through a Medium, 
of which the Parts are susceptible of permanent Vibrations some- 
what slower than the Undulations, their Velocity will be some- 
what lessened by this vibratory Tendency ; and, in the same 
Medium, the more, as the Undulations are more frequent. 
For, as often as the state of the undulation requires a change 
in the actual motion of the particle which transmits it, that 
change will be retarded by the propensity of the particle to 
continue its motion somewhat longer : and this retardation will 
be more frequent, and more considerable, as the difference be- 
tween the periods of the undulation and of the natural vibration 
is greater. 

Corollary. It was long an established opinion, that heat con- 
sists in vibrations of the particles of bodies, and is capable of 
being transmitted by undulations through an apparent va- 
cuum. (Newt. Opt. Qu. 18.) This opinion has been of late 
very much abandoned. Count Rumford, Professor Pictet, and 
Mr. Davy, are almost the only authors who have appeared to 
favour it ; but it seems to have been rejected without any good 
grounds, and will probably very soon recover its popularity. 

Let us suppose that these vibrations are less frequent than 
those of light; all bodies therefore are liable to permanent 
vibrations slower than those of light; and indeed almost all are 
liable to luminous vibrations, either when in a state of ignition, 
or in the circumstances of solar phosphori ; but much less easily, 
and in a much less degree, than to the vibrations of heat. It will 
follow from these suppositions, that the more frequent luminous 
undulations will be more retarded than the less frequent; and 



the Theory of Light and Colours. 33 

consequently, that blue light will be more refrangible than red, 
and radiant heat least of all ; a consequence which coincides 
exactly with the highly interesting experiments of Dr. Her- 
schel. (Phil. Trans, for 1800. p. 284.) It may also be easily 
conceived, that the actual existence of a state of slower vibra- 
tion may tend still more to retard the more frequent undulations, 
and that the refractive power of solid bodies may be sensibly 
increased by an increase of temperature, as it actually appears 
to have been in Euler's experiments. (Acad, de Berlin. 1762. 
p. 328.) 

Scholium. If, notwithstanding, this proposition should appear 
to be insufficiently demonstrated, it must be allowed to be at 
least equally explanatory of the phenomena with any thing that 
can be advanced on the other side, from the doctrine of projec- 
tiles ; since a supposed accelerating force must act in some other 
proportion than that of the bulk of the particles ; and, if we call 
this an elective attraction, it is only veiling under a chemical 
term, our incapacity of assigning a mechanical cause. Mr. 
Short, when he found by observation the equality of the velo- 
city of light of all colours, felt the objection so forcibly, that he 
immediately drew an inference from it in favour of the undula- 
tory system. It is assumed in the proposition, that when light 
is dispersed by refraction, the corpuscles of the refracting sub- 
stance are in a state of actual alternate motion, and contribute 
to its transmission ; but it must be confessed, that we cannot at 
present form a very decided and accurate conception of the 
forces concerned in maintaining these corpuscular vibrations. 



34 Dr. Young's Lecture on 



PROPOSITION VIII. 

When two Undulations, from different Origins, coincide either 
perfectly or very nearly in Direction, their joint effect is a Com- 
bination of the Motions belonging to each. 

Since every particle of the medium is affected by each undu- 
lation, wherever the directions coincide, the undulations can 
proceed no otherwise than by uniting their motions, so that 
the joint motion may be the sum or difference of the separate 
motions, accordingly as similar or dissimilar parts of the undu- 
lations are coincident. 

I have, on a former occasion, insisted at large on the appli- 
cation of this principle to harmonics; (Phil. Trans, for 1800. 
p. 130. ) and it will appear to be of still more extensive utility in 
explaining the phenomena of colours. The undulations which 
are now to be compared are those of equal frequency. When 
the two series coincide exactly in point of time, it is obvious 
that the united velocity of the particular motions must be 
greatest, and, in effect at least, double the separate velocities ; 
and also, that it must be smallest, and if the undulations are of 
equal strength, totally destroyed, when the time of the greatest 
direct motion belonging to one undulation coincides with that 
of the greatest retrograde motion of the other. In intermediate 
states, the joint undulation will be of intermediate strength ; 
but by what laws this intermediate strength must vary, cannot 
be determined without further data. It is well known that a 
similar cause produces in sound, that effect which is called a 
beat ; two series of undulations of nearly equal magnitude co- 
operating and destroying each other alternately, as they coincide 



the Theory of Light and Colours. 35 

more or less perfectly in the times of performing their respective 
motions. 

Corollary i. Of the Colours of striated Surfaces. 

Boyle appears to have been the first that observed the colours 
of scratches on polished surfaces. Newton has not noticed them. 
Mazeas and Mr. Brougham have made some experiments on 
the subject, yet without deriving any satisfactory conclusion. But 
all the varieties of these colours are very easily deduced from 
this proposition. 

Let there be in a given plane two reflecting points very near 
each other, and let the plane be so situated that the reflected 
image of a luminous object seen in it may appear to coincide 
with the points ; then it is obvious that the length of the inci- 
dent and reflected ray, taken together, is equal with respect to 
both points, considering them as capable of reflecting in all 
directions. Let one of the points be now depressed below the 
given plane; then the whole path of the light reflected from it, 
will be lengthened by a line which is to the depression of the 
point as twice the cosine of incidence to the radius. Fig. 2. 

If, therefore, equal undulations of given dimensions be reflected 
from two points, situated near enough to appear to the eye but 
as one, wherever this line is equal to half the breadth of a whole 
undulation, the reflection from the depressed point will so in- 
terfere with the reflection from the fixed point, that the pro- 
gressive motion of the one will coincide with the retrograde 
motion of the other, and they will both be destroyed ; but, when 
this line is equal to the whole breadth of an undulation, the 
effect will be doubled ; and when to a breadth and a half, again 
destroyed ; and thus for a considerable number of alternations ; 
and, if the reflected undulations be of different kinds, they will 
Fa 



3 6 Dr. Young's Lecture on 

be variously affected, according to their proportions to the vari- 
ous length of the line which is the difference between the 
lengths of their two paths, and which may be denominated the 
interval of retardation. 

In order that the effect may be the more perceptible, a num- 
ber of pairs of points must be united into two parallel lines ; 
and, if several such pairs of lines be placed near each other, 
they will facilitate the observation. If one of the lines be made 
to revolve round the other as an axis, the depression below the 
given plane will be as the sine of the inclination ; and, while 
the eye and luminous object remain fixed, the difference of the 
length of the paths will vary as this sine. 

The best subjects for the experiment are Mr. Coventry's 
exquisite micrometers; such of them as consist of parallel lines 
drawn on glass, at the distance of one five hundredth of an. 
inch, are the most convenient. Each of these lines appears 
under a microscope to consist of two or more finer lines, exactly 
parallel, and at the distance of somewhat more than a twentieth, 
of that of the adjacent lines. I placed one of these so as to reflect 
the sun's light at an angle of 45°, and fixed it in such a manner, 
that while it revolved round one of the lines as an axis, I could 
measure its angular motion ; and I found, that the brightest red 
colour occurred at the inclinations io| , 20A , 32°, and 45°; of 
which the sines are as the numbers 1, 2, 3, and 4. At all other 
angles also, when the sun's light was reflected from the sur- 
face, the colour vanished with the inclination, and was equal at 
equal inclinations on either side. 

This experiment affords a very strong confirmation of the 
theory. It is impossible to deduce any explanation of it from 
any hypothesis hitherto advanced ; and I believe it would be 



the Theory of Light and Colours. 37 

difficult to invent any other that would account for it. There 
is a striking analogy between this separation of colours, and the 
production of a musical note by successive echoes from equi- 
distant iron palisades ; which I have found to correspond pretty 
accurately with the known velocity of sound, and the distances 
of the surfaces. 

It is not improbable that the colours of the integuments of 
some insects, and of some other natural bodies, exhibiting in 
different lights the most beautiful versatility, may be found to 
be of this description, and not to be derived from thin plates. 
In some cases, a single scratch or furrow may produce similar 
effects, by the reflection of its opposite edges. 

Corollary ii. Of the Colours of thin Plates. 

When a beam of light falls on two parallel refracting sur- 
faces, the partial reflections coincide perfectly in direction ; and, 
in this case, the interval of retardation, taken between the sur- 
faces, is to their distance as twice the cosine of the angle of 
refraction to the radius. For, in Fig. 3, drawing AB and CD 
perpendicular to the rays, the times of passing through BC and 
AD will be equal, and DE will be half the interval of retarda- 
tion; but DE is to CE as the sine of DCE to the radius. Hence, 
that DE may be constant, or that the same colour may be re- 
flected, the thickness CE must vary as the secant of the angle 
of refraction CED : which agrees exactly with Newton's expe- 
riments; for the correction is perfectly inconsiderable. 

Let the medium between the surfaces be rarer than the sur- 
rounding mediums ; then the impulse reflected at the second 
surface, meeting a subsequent undulation at the first, will render 
the particles of the rarer medium capable of wholly stopping. 



38 Dr. Young's Lecture on 

the motion of the denser, and destroying the reflection, (prop, 
iv.) while they themselves will be more strongly propelled 
than if they had been at rest ; and the transmitted light will be 
increased. So that the colours by reflection will be destroyed, 
and those by transmission rendered more vivid, when the double 
thicknesses, or intervals of retardation, are any multiples of the 
whole breadths of the undulations ; and, at intermediate thick- 
nesses the effects will be reversed; according to the Newtonian 
observations. 

If the same proportions be found to hold good with respect 
to thin plates of a denser medium, which is indeed not impro- 
bable, it will be necessary to adopt the corrected demonstration 
of prop. iv. but, at any rate, if a thin plate be interposed be- 
tween a rarer and a denser medium, the colours by reflection 
and transmission may be expected to change places. 

From Newton's measures of the thicknesses reflecting the 
different colours, the breadth and duration of their respective 
undulations may be very accurately determined ; although it is 
not improbable, that when the glasses approach very near, the 
atmosphere of ether may produce some little irregularity. The 
whole visible spectrum appears to be comprised within the ratio 
of three to five, or a major sixth in music ; ^and the undulations 
of red, yellow, and blue, to be related in magnitude as the 
numbers 8, 7, and 6; so that the interval from red to blue 
is a fourth. The absolute frequency expressed in numbers is 
too great to be distinctly conceived, but it may be better ima- 
gined by a comparison with sound. If a chord sounding the 
tenor c, could be continually bisected 40 times, and should 
then vibrate, it would afford a yellow green light : this being 

denoted by c, the extreme red would be a, and the blue d. 



the Theory of Light and Colours. 39 . 

The absolute length and frequency of each vibration is ex- 
pressed in the table; supposing light to travel in 8| minutes 
500,000,000000 feet. 



Colours. 


Length of an 
Undulation 
in parts of an 
Inch, in Air. 


Number of 
Undulations 
in an Inch. 


Number of Undulations 
in a Second. 


Extreme 


.OOOO266 


37640 


463 millions of millions 


Red - 


.OOOO256 


39180 


482 


Intermediate 


.000024,6 


40720 


501 


Orange 

Intermediate 
Yellow 

Intermediate 


.OOOO24O 

.0000235 

.O0OO227 
.00002ig 


4161O 
42510 
44OOO 
4,5600 


512 
523 
542 
561 (= 2 4 * nearly) 


Green - 

Intermediate 


.0000211 
.OOOO203 


4746o 
49300 


584 
607 


Blue - 


.OOOOI96 


51110 


629 


Intermediate 


.OOOO189 


52910 


652 


Indigo 

Intermediate 


.OOOOI85 
.OOOOlSl 


54°7° 
55240 


665 
680 


Violet - 


.OOOOI74, 


57490 


707 


Extreme - 


.OOOOI67 


59750 


735 



Scholium. It was not till I had satisfied myself respecting all 
these phenomena, that I found in Hooke's Micrographia, a pas- 
sage which might have led me earlier to a similar conclusion. 
" It is most evident that the reflection from the under or fur- 
" ther side of the body, is the principal cause of the production 
" of these colours. — Let the ray fall obliquely on the thin 
" plate, part therefore is reflected back by the first superficies, 
" — part refracted to the second surface, — whence it is reflected 
" and refracted again. — So that, after two refractions and one 



4» Dr. Young's Lecture on 

"reflection, there is propagated a kind of fainter ray — ," and, 
" by reason of the time spent in passing and repassing, — this 
" fainter pulse comes behind the" former reflected " pulse ; so 
" that hereby, (the surfaces being so near together that the eye 
" cannot discriminate them from one,) this confused or duplicated 
" pulse, whose strongest part precedes, and whose weakest fol- 
'" lows, does produce on the retina, the sensation of a yellow. 
" If these surfaces are further removed asunder, the weaker 
" pulse may become coincident with the" reflection of the 
" second," or next following pulse, from the first surface, " and 
" l a gg behind that also, and be coincident with the third, 
" fourth, fifth, sixth, seventh, or eighth — ; so that, if there be 
" a thin transparent body, that from the greatest thinness requi- 
" site to produce colours, does by degrees grow to the greatest 
■" thickness, — the colours shall be so often repeated, as the 
" weaker pulse does lose paces with its primary or first pulse, 
" and is coincident with a" subsequent " pulse. And this, as 
" it is coincident, or follows from the first hypothesis I took of 
" colours, so upon experiment have I found it in multitudes of 
"instances that seem to prove it." (P. 65 — 67.) This was 
printed about seven years before any of Newton's experiments 
were made. We are informed by Newton, that Hooke was 
afterwards disposed to adopt his " suggestion" of the nature of 
colours ; and yet it does not appear that Hooke ever applied that 
improvement to his explanation of these phenomena, or inquired 
into the necessary consequence of a change of obliquity, upon 
his original supposition, otherwise he could not but have dis- 
covered a striking coincidence with the measures laid down by 
Newton from experiment. All former attempts to explain the 
colours of thin plates, have either proceeded on suppositions 



the Theory of Light and Colours. 4.1 

which, like Newton's, would lead us to expect the greatest irre- 
gularities in the direction of the refracted rays ; or, like Mr. 
Michell's, would require such effects from the change of the 
angle of incidence, as are contrary to the effects observed ; or 
they are equally deficient with respect to both these circum- 
stances, and are inconsistent with the most moderate attention 
to the principal phenomena. 

Corollary hi. Of the Colours of thick Plates, 
When a beam of light passes through a refracting surface, 
especially if imperfectly polished, a portion of it is irregularly 
scattered, and makes the surface visible in all directions, but 
most conspicuously in directions not far distant from that of 
the light itself: and, if a reflecting surface be placed parallel to 
the refracting surface, this scattered light, as well as the prin- 
cipal beam, will be reflected, and there will also be a new dis- 
sipation of light, at the return of the beam through the refracting 
surface. These two portions of scattered light will coincide in 
direction ; and, if the surfaces be of such a form as to collect 
the similar effects, will exhibit rings of colours. The interval 
of retardation is here, the difference between the paths of the 
principal beam and of the scattered light between the two sur- 
faces ; of course, wherever the inclination of the scattered light 
is equal to that of the beam, although in different planes,. the 
interval will vanish, and all the undulations will conspire. At 
other inclinations, the interval will be the difference of the 
secants from the secant of the inclination or angle of refraction 
of the principal beam. From these causes, all the colours of 
concave mirrors observed by Newton and others are necessary 
consequences: and it appears that their production, though 

MDCCCII. Gr 



4* Dr. Young's Lecture on 

somewhat similar, is by no means, as Newton imagined, iden- 
tical with the production of those of thin plates. 

Corollary iv. Of Blackness. 
In the three preceding corollaries, we have considered the 
refracting and reflecting substances as limited by a mathema- 
' tical surface; but this is perhaps never physically true. The 
ethereal atmospheres may extend on each side the surface as 
far as the breadth of one or more undulations ; and, if they be 
supposed to vary equally in density at every part, the partial 
reflections from each of the infinite number of surfaces, where 
the density changes, will very much interfere with each other, 
and destroy a considerable portion of the reflected light, so that 
the substance may become positively black; and this effect may 
take place in a greater or less degree, as the density of the 
ethereal atmosphere varies more or less equably; and, in some 
cases, particular undulations being more affected than others, 
a tinge of colour may be produced. Accordingly, M. Bouguer 
has observed a considerable loss of light, and in some instances 
a tinge of colour, in total reflections at the surface of a rarer 
medium. 

Corollary, v. Of Colours by Inflection. 
Whatever may be the cause of the inflection of light passing 
through a small aperture, the light nearest its centre must be 
the least diverted, and the nearest to its sides the most : ano- 
ther portion of light falling very obliquely on the margin of the 
aperture, will be copiously reflected in various directions ; some 
of which will either perfectly or very nearly coincide in direc- 
tion with the unreflected light, and, having taken a circuitous 



the Theory of Light and Colours. 43 

route, will so interfere with it, as to cause an appearance of 
colours. The length of the two tracks will differ the less, as 
the direction of the reflected light has been less changed by its 
reflection, that is, in the light passing nearest to the margin ; so 
that the blues will appear in the light nearest the shadow. The 
effect will be increased and modified, when the reflected light 
falls within the influence of the opposite edge, so as to interfere 
with the light simply inflected by that also. 

But, in order to examine the consequences more minutely, it 
will be convenient to suppose the inflection caused by an ethereal 
atmosphere, of a density varying as a given power of the dis- 
tance from a centre, as in the eighth proposition of the last 
Bakerian Lecture. (Phil, Trans, for 1801, p. 83.) Putting 
r = 3, and x =£, I have constructed a diagram, (Fig. 4,,) which 
shows, by the two pairs of curves, the relative position of the re- 
flected and unreflected portions of any one undulation at two 
successive times, and also, by shaded lines drawn across, the parts 
where the intervals of retardation are in arithmetical progression, 
and where similar colours will be exhibited at different distances 
from the inflecting substance. The result fully agrees with the 
observations of Newton's third book, and with those of later 
writers. But I do not consider it as quite certain, until further 
experiments have been made on the inflecting power of dif- 
ferent substances, that Dr. Hooke's explanation of inflection, 
by the tendency of light to diverge, may not have some preten- 
sions to truth. I am sorry to be obliged to recall here the assent 
which, at first sight, I was induced to give to a supposed im- 
provement of a late author. (Phil. Trans, for 1800, p. 128.) 

Scholium. In the construction of the diagram, it becomes ne- 
cessary to find the time spent by each ray in its passage, 
G 2 



44 Dr. Young's Lecture on 

Since the velocity was denoted by x~ ~, on the supposition of a 
projectile, it will be as x ~ on the contrary supposition, (Phil. 
Trans, for 1801, p. 27. Schol. 2. Prop. I.) and the fluxion of the 

distance described being 7==. that of the time will be * >/ -'J'-' 
or—. i- , of which the fluent is 737 --7- 1/1— yy. 

t — r yy.Vi—yy y 

Therefore, with the radius x l ~~, describe a circle concentric 
with the surfaces of the inflecting atmosphere, then the angle 
described by the ray during its passage through the atmosphere, 
will always be to the angle subtended by the line cut off by 
this circle from the incident ray produced, in the ratio of r to 

r 1; and the time spent in this passage, will be in the same 

ratio to the time that would have been spent in describing this 
intercepted portion with the initial velocity. For y, being equal 
to sx^~\ is the sine of the inclination of the incident ray to the 
radius, where it meets this circle ; therefore, by the proposition 
quoted, the angle described is in a given ratio to the angle at 
the centre, which is the difference of the inclinations. Making 
j: 1 --ror— radius, the sine, instead of y, becomes s, and the co- 

sine s/ — — ss, or — s/i — yy, and, when y == ss, \/i — ss; 
therefore the line intercepted is to the difference of the fluents 
as r to r — 1. (See also Young's Syllabus, Art. 372.) 

PROPOSITION IX. 

Radiant Light consists in Undulations of the luminiferous Ether. 
This proposition is the general conclusion from all the pre- 
ceding ; and it is conceived that they conspire to prove it in as 
satisfactory a manner as can possibly be expected from the 



the Theory of Light and Colours. 45 

nature of the subject. It is clearly granted by Newton, that 
there are undulations, yet he denies that they constitute light; 
but it is shown in the three first Corollaries of the last Proposi- 
tion, that all cases of the increase or diminution of light are 
referable to an increase or diminution of such undulations, and 
that all the affections to which the undulations would be liable, 
are distinctly visible in the phenomena of light; it may there- 
fore be very logically inferred, that the undulations are light. 

A few detached remarks will serve to obviate some objections 
which may be raised against this theory. 

i. Newton has advanced the singular refraction of the Ice- 
land crystal, as an argument that the particles of light must be 
projected corpuscles; since he thinks it probable that the dif- 
ferent sides of these particles must be differently attracted by 
the crystal, and since Huygens has confessed his inability to 
account in a satisfactory manner for all the phenomena. But, 
contrarily to what might have been expected from Newton's 
usual accuracy and candour, he has laid down a new law for 
the refraction, without giving a reason for rejecting that of 
Huygens, which Mr. Hauy has found to be more accurate than 
Newton's; and, without attempting to deduce from his own 
system any explanation of the more universal and striking effects 
of doubling spars, he has omitted to observe that Huygens's 
most elegant and ingenious theory perfectly accords with these 
general effects, in all particulars, and of course derives from 
them additional pretensions to truth: this he omits, in order to 
point out a difficulty, for which only a verbal solution can be 
found in his own theory, and which will probably long remam 
unexplained by any other. 

2 Mr. Michell has made some experiments, which appear 
to show that the rays of light have an actual momentum, by 



4^ Dr. Young's Lecture on 

means of which a motion is produced when they fall on a thin 
plate of copper delicately suspended. (Priestley's Optics.) 
But, taking for granted the exact perpendicularity of the plate, 
and the absence of any ascending current of air, yet since, in 
every such experiment, a greater quantity of heat must be com- 
municated to the air at the surface on which the light falls than 
at the opposite surface, the excess of expansion must necessarily 
produce an excess of pressure on the first surface, and a very 
perceptible recession of the plate in the direction of the light. 
Mr. Bennet has repeated the experiment, with a much more 
sensible apparatus, and also in the absence of air ; and very justly 
infers from its total failure, an argument in favour of the undu- 
latory system of light. (Phil. Trans, for 1792, p. 87.) For, 
granting the utmost imaginable subtility of the corpuscles of 
light, their effects might naturally be expected to bear some 
proportion to the effects of the much less rapid motions of the 
electrical fluid, which are so very easily perceptible, even in 
their weakest states. 

3. There are some phenomena of the light of solar phosphori, 
which at first sight might seem to favour the corpuscular sys- 
tem; for instance, its remaining many months as if in a latent 
state, and its subsequent re-emission by the action of heat. 
But, on further consideration, there is no difficulty in supposing 
the particles of the phosphori which have been made to vibrate 
by the action of light, to have this action abruptly suspended 
by the intervention of cold, whether as contracting the bulk of 
the substance or otherwise ; and again, after the restraint is 
removed, to proceed in their motion, as a spring would do which 
had been held fast for a time in an intermediate stage of its vibra- 
tion ; nor is it impossible that heat itself may, in some circum- 
stances, become in a similar manner latent. (Nicholson's 



the Theory of Light and Colours, 4,7 

Journal. Vol. II. p. 399. ) But the affections of heat may perhaps 
hereafter be rendered more intelligible to us ; at present, it seems 
highly probable that light differs from heat only in the frequency 
of its undulations or vibrations ; those undulations which are 
within certain limits, with respect to frequency, being capable of 
affecting the optic nerve, and constituting light; and those which 
are slower, and probably stronger, constituting heat only ; that 
light and heat occur to us, each in two predicaments, the vibratory 
or permanent, and the undulatory or transient state; vibratory 
light being the minute motion of ignited bodies, or of solar phos- 
phor!, and undulatory or radiant light the motion of the ethereal 
medium excited by these vibrations; vibratory heat being a motion 
to which all material substances are liable, and which is more or 
less permanent ; and undulatory heat that motion of the same 
ethereal medium, which has been shown by Mr. King, (Mor- 
sels of Criticism. 1786. p. 99,) and M. Pictet, (Essais de Phy- 
sique. 1790,) to be as capable of reflection as light, and by Dr. 
Herschel to be capable of separate refraction. (Phil. Trans, for 
1800. p. 284.) How much more readily heat is communicated 
by the free access of colder substances, than either by radiation 
or by transmission through a quiescent medium, has been shown 
by the valuable experiments of Count Rumford. It is easy to 
conceive that some substances, permeable to light, may be unfit 
for the transmission of heat, in the same manner as particular 
substances may transmit some kinds of light, while they are 
opaque with respect to others. 

On the whole it appears, that the few optical phenomena 
which admit of explanation by the corpuscular system, are 
equally consistent with this theory ; that many others, which 
have long been known, but never understood, become by these 
means perfectly intelligible; and that several new facts are 



48 Dr. Young's Lecture, &c. 

found to be thus only reducible to a perfect analogy with other 
facts, and to the simple principles of the undulatory system. It is 
presumed, that henceforth the second and third books of New- 
ton's Optics will be considered as more fully understood than 
the first has hitherto been ; but, if it should appear to impartial 
judges, that additional evidence is wanting for the establishment 
of the theory, it will be easy to enter more minutely into the 
details of various experiments, and to show the insuperable dif- 
ficulties attending the Newtonian doctrines, which, without 
necessity, it would be tedious and invidious to enumerate. The 
merits of their author in natural philosophy, are great beyond all 
contest or comparison ; his optical discovery of the composition 
of white light, would alone have immortalised his name; and the 
very arguments which tend to overthrow his system, give the 
strongest proofs of the admirable accuracy of his experiments. 
Sufficient and decisive as these arguments appear, it cannot 
be superfluous to seek for further confirmation; which may with 
considerable confidence be expected, from an experiment very in- 
geniously suggested by Professor Robison, on the refraction of the 
light returning to us from the opposite margins of Saturn's ring; 
for, on the corpuscular theory, the ring must be considerably 
distorted when viewed through an achromatic prism : a similar 
distortion ought also to be observed in the disc of Jupiter; but, 
if it be found that an equal deviation is produced in the whole 
light reflected from these planets, there can scarcely be any re- 
maining hope to explain the affections of light, by a comparison 
with the motions of projectiles. 



Sirvw 
■$. I. Ib.O